1. Technical Field
The present application is based on Japanese Patent Application No. 2012-198472 filed on Sep. 10, 2012, the entire contents of which are hereby incorporated by reference.
The present invention relates to a group III nitride semiconductor crystal, particularly relates to the group III nitride semiconductor crystal having a low carrier concentration, and further relates to a group III nitride semiconductor substrate, a group III nitride semiconductor freestanding substrate, a nitride semiconductor device, and a rectifier diode.
2. Description of Related Art
The group III nitride semiconductor is composed of the group III nitride semiconductor crystal. For example, a gallium nitride semiconductor (GaN semiconductor), etc., is given as the group III nitride semiconductor. The GaN semiconductor has high band gap, high electron mobility, high saturation electron speed, and high breakdown field, compared with a semiconductor such as silicon. Therefore, attention is paid to the GaN semiconductor as a material for the use of a power device such as a diode and a transistor, etc.
The power device has a breakdown voltage layer, and is used for equipment of high voltage and large current. N-type group III nitride semiconductor is used for the breakdown voltage layer, and high breakdown voltage is required for the group III nitride semiconductor. Further, from a viewpoint of energy-saving and low heat generation, the group III nitride semiconductor is required to have a low electric resistance (on-resistance) in on-state, which is a low on-resistance.
The breakdown voltage and the on-resistance are determined by a carrier concentration of the group III nitride semiconductor. The breakdown voltage becomes high in a high breakdown voltage state as a carrier concentration becomes low. Meanwhile, the on-resistance becomes low in a low-on-resistance state as the carrier concentration is high. Namely, the breakdown voltage and the on-resistance are set in a relation of trade-off.
In order to obtain the high breakdown voltage and the low on-resistance in the power device, the n-type nitride semiconductor having a low carrier concentration is required as the breakdown voltage layer. For example, the carrier concentration is required to be 1.0E+16 cm−3 or less.
The carrier concentration is controlled by the concentration of a donor-type impurity (such as Si, etc.) with which the n-type group III nitride semiconductor is doped. In then-type group III nitride semiconductor, a carrier is generated by the donor-type impurity, and therefore the carrier concentration corresponds to the concentration of the donor-type impurity. However, an acceptor-type defect is mixed into the n-type group III nitride semiconductor, and therefore the carrier concentration is reduced by a compensation of the acceptor-type defect. The compensation means the reduction of the carrier concentration because a part of the carrier generated by the donor-type impurity is captured by the acceptor-type defect. The reduction of the carrier concentration by compensation, corresponds to the concentration of the mixed acceptor-type defect. Namely, the carrier concentration corresponds to the concentration of the donor-type impurity after compensation, which is the concentration after subtracting the concentration of the acceptor-type defect from the concentration of the donor-type impurity.
An acceptor-type impurity and an acceptor-type specific defect are known as the acceptor-type defect.
The acceptor-type impurity is an unintended impurity, which is the impurity inevitably mixed into a crystal when the n-type group III nitride semiconductor is crystal-grown. The carrier concentration is reduced by the acceptor-type impurity which compensates the doped donor-type impurity. For example, carbon (C), etc., is given as the acceptor-type impurity, wherein the carbon is derived from trimethylgallium ((CH3)3Ga) which is a Ga source used for forming the GaN semiconductor for example (for example, see non-patent document 1).
The acceptor-type specific defect is one kind of a lattice defect in a crystal structure of the group III nitride semiconductor, and is generated by lack of Ga, etc., in the GaN semiconductor. The carrier concentration is reduced by the acceptor-type specific defect by compensating the donor-type impurity.
In manufacturing the n-type group III nitride semiconductor, the carrier concentration is controlled by suitably selecting the donor-type impurity in consideration of the reduction of the carrier concentration due to compensation of the acceptor-type defect.    [Non-patent document 1] J. Cryst. Growth, 298,871 (S. Hashimoto et al. 2007)
As described above, in order to obtain the high breakdown voltage and the low on-resistance in the n-type group III nitride semiconductor, the carrier concentration is required to be reduced to 1.0E+16 cm−3 or less for example. In order to reduce the carrier concentration, it can be considered that an addition amount of the donor-type impurity is reduced, which generates the carrier, and the concentration thereof is reduced.
However, it is difficult to realize the low carrier concentration even in a case of the low concentration of the donor-type impurity when manufacturing the n-type group III nitride semiconductor, and for example, it is difficult to obtain the n-type group III nitride semiconductor having the carrier concentration of 1.0E+15 cm−3 or less. Further, since the low carrier concentration is hardly realized, it is difficult to obtain the n-type group III nitride semiconductor of high breakdown voltage and low on-resistance.